Based on the achievement of synthesis of LSCO nanotubes, a nanotube gas sensor was fabricated with microelectromechanical system technology and its NH3sensing characteristics were invest
Trang 1bKey Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, People’s Republic of China
cCenter of Nanoscience and Nanotechnology, Wuhan University, Wuhan 430072, People’s Republic of China
a r t i c l e i n f o
Article history:
Received 26 November 2007
Received in revised form 13 April 2008
Accepted 14 April 2008
Available online 30 April 2008
Keywords:
LSCO nanotubes
Gas sensors
Fast response
a b s t r a c t
La0.5Sr0.5CoO3−ı(LSCO) nanotubes were synthesized by using a porous anodic aluminum oxide (AAO) template from a sol–gel solution Based on the achievement of synthesis of LSCO nanotubes, a nanotube gas sensor was fabricated with microelectromechanical system technology and its NH3sensing characteristics were investigated Capacitance of LSCO nanotubes was changed by two orders of magnitude within several seconds of exposure to NH3molecules at room temperature The detection limit of the LSCO nanotube sensor was several ppm, and the typical response and recovery time of the sensor at room temperature was only several seconds Our results demonstrate the potential application of LSCO nanotubes for fabricating
a highly sensitive and fast response gas sensor
© 2008 Elsevier B.V All rights reserved
1 Introduction
Recently, one-dimensional (1-D) nanostructures, including
semiconducting carbon nanotubes [1,2], functionalized carbon
nanotubes [3,4], Si nanowires [5,6], ZnO nanowires [7], V2O5
nanowires[8], In2O3nanowires[9,10], WO3nanowires[11], SnO2
nanowires[12], and metal nanowires[13,14], have been
demon-strated as effective ultrasensitive chemical and biological sensors
because of their high surface-to-volume ratio and their unique
electrical properties These features may enable a sensitivity high
enough to charged analytes so that single molecule detection
becomes possible[15] In addition, the direct conversion that from
chemical information into electrical signal can take advantage of
existing low-power microelectronic technology and lead to
minia-turized sensor devices
The recovery time is a very important parameter for gas sensors
Most of the nanotube based gas sensors have slow recovery time
ranging from several minutes to several hours[1,11,12,16], which
limits the practical application of these sensors Some assistant
methods, such as UV irradiation and high voltage pulse[11,16,17]
are used to decrease the recovery time, but these assistant methods
also make the sensors inconvenient to use In this letter we report
a fast recover gas sensor based on LSCO nanotubes Our devices
exhibit a large response to NH3 at room temperature Moreover,
∗ Corresponding author at: Department of Physics, Wuhan University, Wuhan
430072, People’s Republic of China.
E-mail address:wliu.whu@gmail.com (X.-z Zhao).
Response time as short as several seconds has also been achieved, which is far better than the results previously obtained[1,11,12,16]
2 Experiment methods
2.1 AAO membrane preparation
High-purity aluminum sheets (99.999%, 20 mm× 10 mm) were used in this experiment Prior to anodization, the metal surfaces were degreased, etched in an alkaline solution, rinsed in distilled water, and electropolished to achieve a smooth surface It was nec-essary to immerse the samples in a concentrated acid or alkaline solution for several minutes to remove the oxide layer formed during the electropolishing process All samples were rinsed in dis-tilled water again and then transferred to a nitrogen environment The resultant clean aluminum samples were anodized at constant potential in 0.3 M oxalic acid (C2H2O4) (40 V, 4◦C, Pt sheet as a counter electrode) The anode was then immersed in an aqueous solution of 0.6 M H3PO4 and 0.15 M H2CrO4 at 60◦C for 10 h to remove the alumina layer Subsequently, the Al sheet was rean-odized for 20 h under the same condition again and became an AAO template with highly ordered nanoporous arrays
2.2 LSCO sol preparation
The LSCO sol were prepared from the starting materials
of lanthanum acetate (La(CH3COO)3·1.5H2O), strontium acetate (Sr(CH3COO)2·0.5H2O) and cobalt acetate (Co(CH3COO)2·4H2O) The starting materials were mixed at a molar ratio of La:Sr:Co 0925-4005/$ – see front matter © 2008 Elsevier B.V All rights reserved.
Trang 2Fig 1 Apparatus for gas sensing test.
= 1− x:x:1 and dissolved in heated acetic acid and deionized
water; acetylacetone (CH3COCH2COCH3) was added to stabilize the
solution at a the volume ratio of CH3COCH2COCH3/H2O = 1:1)
For-mamide (HCONH2) was also added to the system at a volume ratio
of HCONH2/H2O = 1:3) in order to avoid cracking during heating
The concentration of solution was diluted to 0.3 M Co
2.3 LSCO nanotubes preparation
The alumina template membrane was dipped into the sol for
a desired period of time and then removed, the excess sol on the
membrane surface was wiped off using a laboratory tissue, followed
by drying under vacuum at 100◦C for 24 h The membrane surface
was carefully wiped again to remove salts crystallized on the
sur-face and heated at 700◦C for 4 h in open air, resulting in formation
of arrays of LSCO nanotubes in the inside of the pores of the AAO
template
2.4 Characterization
The morphologies of the LSCO nanowires were characterized by
a scanning electron microscope (SEM, Sirion FEG, FEI)
For the capacitance measurement, a pair of interdigitated
elec-trodes (IDE) was fabricated using a conventional photolithographic
method with a finger width of 8m and a gap size of 8 m The IDE
fingers were made by sputtering 20-nm Ti and 40-nm Pt on a layer
of silicon dioxide (SiO2) thermally grown on top of a silicon wafer
The suspension of LSCO was strewn on the IDE fingers
2.5 Measurement of sensing characteristics
Gas-sensing experiments were carried out using a capacitance
measurement system, as represented inFig 1 During the
experi-ment, an LSCO nanotube gas sensor was placed in a sealed chamber
Diluted NH3in a carrier gas of air flowed through the sealed
cham-ber while we are monitoring the capacitance and dielectric loss
of the LSCO nanotubes All measurements were operated at room
temperature The capacitance and dielectric loss responses during
testing were monitored by a precision impedance analyzer (Agilent
4294a)
3 Results and discussions
Fig 2illustrates the SEM images of LSCO nanotubes on
micro-electrodes It can be seen in Fig 2, the diameter of the LSCO
Fig 2 SEM image of LSCO nanotubes on Au microelectrodes.
nanotubes is about 50 nm, which is similar to the pore diameter
of the template These LSCO nanotubes are put over two Pt/Ti elec-trodes
The capacitive NH3sensing properties of LSCO nanotubes were measured at room temperature by placing the device in a testing chamber Exposure to NH3molecules increased the capacitance of the sensor (Fig 3) It has been found that there exists a dependency
of the capacitance on the applied signal frequency Clearly, the device’s response to NH3gas was more sensitive at lower frequency
A capacitance change of about three orders of magnitude had been achieved to 1000 ppm NH3at a frequency of 100 Hz However, the noise at low frequency was not neglected Therefore 10,000 Hz was chosen as the applied signal frequency to obtain enough sensitivity and negligible noise The dielectric loss versus frequency in dif-ferent concentrations of NH3 is depicted inFig 4 When the NH3
concentration increased, the resonant frequency shifts from low to high frequency, which is corresponding to faster ion transport in high NH3concentration of NH3
Typical response curve obtained with different steps of NH3 con-centration variation at the 10,000 Hz frequency is reported inFig 5; the measurements were performed at room temperature After 0.5% NH3was induced, the capacitance of the PAA sample increased
by about three orders of magnitude (Fig 5a) And then the NH3 con-centration decreased by 20% in every step The capacitance of the device decreased along with the decreasing NH3concentration The capacitance and dielectric loss variation versus NH3 concen-tration was measured for the same device and the plots are shown
Fig 3 Capacitance of LSCO nanotubes versus frequency at different NH3
Trang 3concen-Fig 4 Dielectric loss of LSCO nanotubes versus frequency at different NH3
concen-trations.
Fig 5 (a) Capacitance and (b) dielectric loss responses to the stepwise decreases of
the NH 3 concentration at a frequency of 10,000 Hz.
inFig 6 From the plot we can see that the capacitance of the device
increases along with the increasing NH3concentration It is clearly
evident that the LSCO nanotube sensor exhibits a greater
sensitiv-ity towards NH3 The detection limit of the LSCO nanotube sensor
is several ppm The capacitance varied with the NH3concentration
monotonically but nonlinearly, while the dielectric loss was
non-monotonically related to the NH3 concentration When the NH3
Fig 6 The plots of (a) and (b) dielectric loss variation as a function of NH3
concen-Fig 7 (a) Capacitance and (b) dielectric loss changes at 10,000 Hz upon exposure
to NH 3 of 10–200 ppm.
concentration is lower than 200 ppm, the dielectric loss increased with increasing NH3concentration However, in the range from 200
to 1000 ppm, the dielectric loss decreased with increasing NH3 con-centration These results matched well with the data inFig 4 In Fig 4, the points on the dashed line correspond to dielectric losses
at 10,000 Hz We can see that the dielectric loss peak is obtained to about 200 ppm of NH3, which corresponds to the peak in curve (b)
inFig 6
To understand the sensitivity in low NH3 concentration, the dynamics gas response of the LSCO nanotube sensor to low con-centrations of NH3is shown inFig 7 Curves (a) and (b) represent how the capacitance and dielectric loss responses to NH3of 200, 50,
30, 20 and 10 ppm, respectively The variation amplitudes at vari-ous NH3concentrations could be reflected clearly by the function of
device sensitivity We define capacitance response (SC) as the ratio
SC= ((CA− CG)/CA)× 100%, where CArepresents the capacitance in
air and CGthe capacitance in gas The dielectric loss response (SD)
is defined as the ratio SD= ((DA− DG)/DA)× 100%, where DA
rep-resents the dielectric loss in air and DGthe dielectric loss in gas
We can see that the capacitance response is 126, 61, 33, 16 and 12% to NH3of 400, 100, 50, 30 and 20 ppm respectively, while the dielectric loss response is 337, 220, 168, 112 and 80% The dielectric loss response is much higher than the capacitance response These results are in good accordance with the data inFig 6
Response time is one of the most important parameters for all sensors Generally, this property of a gas sensor mainly depends upon the response time at low gas concentrations The room-temperature response and recovery time of the LSCO nanotube sensor at low NH3concentrations are presented inFig 7 The exper-imental data showed that about only several seconds was needed for the capacitance to reach 90% of the total variation values dur-ing both NH3adsorption and desorption processes, These results were far better than most of other 1-D nanostructrued materials [1,11,12,16]
The sensing mechanism of LSCO nanotubes to NH3 was sug-gested to be related with the change of the overall dielectric constant or a surface reaction process The capacitance and dielec-tric loss variation with NH3of LSCO nanotubes may have originated mainly from the NH3molecule adsorption on the walls of LSCO nan-otubes, replacement of the air in the voids of the nanopores by NH3 vapors, and possible surface reaction The fast response and recov-ery time may be due to the physical adsorption of NH3on the LSCO nanotube surface
Trang 4and Development Program (973 Project) of China (Grant No.
2006CB932305)
References
[1] E.S Snow, F.K Perkins, E.J Houser, S.C Badescu, T.L Reinecke, Chemical
detec-tion with a single-walled carbon nanotube capacitor, Science 307 (2005)
1942–1945.
[2] J Kong, N.R Franklin, C.W Zhou, M.G Chapline, S Peng, K.J Cho, H.J Dai,
Nan-otube molecular wires as chemical sensors, Science 287 (2000) 622–625.
[3] K Besteman, J.O Lee, F.G.M Wiertz, H.A Heering, C Dekker, Enzyme-coated
carbon nanotubes as single-molecule biosensors, Nano Lett 3 (2003) 727–730.
[4] J Kong, M.G Chapline, H.J Dai, Functionalized carbon nanotubes for molecular
hydrogen sensors, Adv Mater 13 (2001) 1384–1386.
[5] W.W Chen, H Yao, C.H Tzang, J.J Zhu, M.S Yang, S.T Lee, Silicon nanowires for
high-sensitivity glucose detection, Appl Phys Lett 88 (2006) 213104–213106.
[6] Z Li, Y Chen, X Li, T.I Kamins, K Nauka, R.S Williams, Sequence-specific
label-free DNA sensors based on silicon nanowires, Nano Lett 4 (2004) 245–247.
[7] Q Wan, Q.H Li, Y.J Chen, T.H Wang, X.L He, J.P Li, C.L Lin, Fabrication and
ethanol sensing characteristics of ZnO nanowire gas sensors, Appl Phys Lett.
84 (2004) 3654–3656.
[8] H.Y Yu, B.H Kang, U.H Pi, C.W Park, S.Y Choi, G.T Kim, V 2 O 5
nanowire-based nanoelectronic devices for helium detection, Appl Phys Lett 86 (2005)
253102–253104.
[9] D.H Zhang, Z.Q Liu, C Li, T Tang, X.L Liu, S Han, B Lei, C.W Zhou, Detection of
NO 2 down to ppb levels using individual and multiple In 2 O 3 nanowire devices,
Nano Lett 4 (2004) 1919–1924.
86 (2005) 123510–123512.
Biographies
Wei Liu received his MS degree in physics at Wuhan University in 2003 and presently
is a graduate student for her PhD degree in physics at Wuhan University Her field
of interest is nanomaterials and Lab on a Chip.
Sheng Wang received his MS degree in physics at Wuhan University in 2005 and
presently is a graduate student for her PhD degree in physics at Wuhan University His field of interest is nanomaterials and devices.
Yu Chen received his BS degree in physics at Wuhan University in 2007 and presently
is a graduate student for his MS degree in physics at Wuhan University.
Meiya Li received his PhD in physics at Beijing University in China (1997) and
presently is a professor in Department of Physics of Wuhan University His current fields of interest are nanomaterials.
Guojia Fang received his PhD in physics at Huazhong University of Science and
Technology in China (2000) and presently is a professor in Department of Physics of Wuhan University His current fields of interest are nanomaterials
Xing-Zhong Zhao received his PhD in physics at University of Science and
Technology of Beijing in China (1989) and presently is a professor in Depart-ment of Physics of Wuhan University His current fields of interest are Lab on a Chip.